Why is the firing end appearance of a spark plug important?

A Because the firing end appearance reflects the suitability of the spark plug as well as the condition of the engine.

These are the three basic standards for evaluating a spark plug.
Spark plug tip temperature and firing end appearance

The border-line between the fouling and optimum operating regions (500 °C) is called the spark plug self-cleaning temperature.
It is at this temperature that accumulated carbon deposits are burnt off.